This invention relates to alkaline galvanic cells, and particularly to nickel zinc alkaline cells which have a zinc oxide negative electrode, an alkaline electrolyte, and a nickel hydroxide positive electrode.
A number of prior art patents teach various approaches to improve the performance of alkaline cells, including alkaline cells having pasted nickel hydroxide positive electrodes, alkaline cells having rechargeable zinc negative electrodes, various electrolyte formulations, and so on. Also, it is well known that the performance of pasted nickel hydroxide electrodes, for example, can be improved by the provision of light weight current collectors, and by increasing the packing density of the nickel hydroxide active material, thereby improving the achievable energy density of the electrodes.
It is also well known to provide conductive diluents such as nickel powder and cobalt oxide to improve active material utilization. For example, YAO et al U.S. Pat. No. 5,759,718, issued Jun. 2, 1998, teaches the provision of alkaline storage batteries wherein cobalt hydroxide having specific characteristics is added to the positive electrode active material.
Likewise, BOGAUCHI et al, U.S. Pat. No. 5,489,314, issued Feb. 6, 1996, teach an alkaline battery having a nickel plate on which cobalt oxyhydroxide is previously formed on the surfaces of the positive active material powder.
In the above patents, as the CoO material slowly dissolves during the initial charge, and is oxidized to conductive insoluble CoOOH, a conductive network is set up in situ within the structure of the nickel hydroxide electrode.
In some instances, an external chemical oxidant is used to promote the reaction, as is also taught by the Bogauchi et al patent.
The performance of rechargeable zinc electrodes in alkaline electrolytes is the subject, for example, of JONES U.S. Pat. No. 4,358,517, issued Nov. 9, 1982. This patent teaches a nickel-zinc cell where the zinc electrode has a copper grid and an active material that comprises zinc-rich particles, calcium-rich particles, and an entanglement of cellulose fibres. Lead compounds may also be added to improve turn around efficiency and to reduce water loss.
The use of buffered electrolytes is also contemplated, for improvement of rechargeable zinc cells. ADLER et al U.S. Pat. No. 5,453,336, issued Sep. 26, 1995, teaches the use of an electrolyte that contains one or more hydroxides of an alkali metal, one or more fluorides of an alkali metal, and one or more carbonates of an alkali metal.
Another patent which teaches a ternary electrolyte for secondary electrochemical cells is CARLSON U.S. Pat. No. 4,273,841, issued Jun. 16, 1981, which teaches an aqueous alkaline solution having potassium hydroxide, potassium fluoride, and potassium phosphate.
EISENBERG U.S. Pat. No. 4,224,391 issued Sep. 23, 1980, and U.S. Pat. No. 5,215,836 issued Jun. 1, 1993, each describe electrolyte formulations that employ mixtures of potassium hydroxide and boric, phosphoric, or arsenic acids.
As noted, pasted nickel hydroxide electrodes can have their performance improved by the addition of conductive diluents to improve active material utilization, and such as to establish a conductive insoluble CoOOH network in situ within a pasted nickel hydroxide electrode.
However, the problem of low positive electrode efficiency is exacerbated, sometimes significantly, when their use in nickel zinc batteries is considered, because of the electrolyte requirements of the zinc electrode.
Of course, it is well known that performance inhibiting disfigurement or rearrangement of zinc electrodes can occur during the cycling process of rechargeable zinc electrodes in alkaline electrolytes. Such disfigurement can be minimized in more dilute alkali hydroxide solutions.
The Jones Patent, noted above, appears to be somewhat effective in extending cycle life of the cell by the addition of calcium hydroxide to the zinc electrode.
It has also been noted that buffered electrolytes with or without fluoride additions may also result in increased zinc electrode lifespan. They are particularly described in the Adler et al patent, noted above, where a mixture of alkaline electrolyte having a strength of 2M to 12M is combined with a carbonate of 0.5M to 4M, and a fluoride of 0.5M to 4M.
The Carlson Patent, noted above, describes a mixture that employs 5% to 10% of hydroxide, 10% to 20% of phosphate, and 5% to 15% of fluoride.
However, an undesirable feature of all of these approaches is the diminished utilization of nickel hydroxide as the electrolyte is diluted below the optimal 30% by weight of potassium hydroxide.
It is always to be noted that nickel is one of the more expensive components of a nickel zinc cell, so that it is important to maximize its efficiency during charge and discharge. This problem has been addressed by such as the addition of calcium fluoride so as to improve high temperature charge acceptance, as discussed in Japanese Patent JP5290840A2.
Also, nickel hydroxide particle modification has been addressed with the use of yttrium, scandium, or lanthanide hydroxide, oxide, or fluoride, as taught in Japanese Patents JP11007949 and JP173614A2.
The present inventor has unexpectedly found that small additions of fluoride to the nickel hydroxide paste which is employed in the production of positive electrodes for nickel zinc cells has resulted in significant improvements in active material utilization.
To that end, therefore, the present invention provides a nickel zinc alkaline cell having a zinc oxide negative electrode supported on a conductive substrate, an alkaline electrolyte, and a positive electrode having nickel hydroxide paste supported on a conductive substrate.
The positive electrode further comprises 0.01% to 1% by weight of a fluoride salt.
The fluoride salt is a salt of a metal which is chosen from the a group consisting of: potassium, sodium, lithium, rubidium, caesium, a group II metal, a group III metal, a d-block transition metal, an f-block lanthanide, and mixtures thereof.
Typically, the fluoride salt is potassium fluoride.
Also, typically the potassium fluoride is used in the amount of about 0.1% by weight of the nickel hydroxide.
The nickel hydroxide positive electrode may further comprise a conductivity enhancement additive which is chosen from the group consisting of: 2% to 3% by weight of nickel metal powder, 2% to 3% by weight of cobalt metal powder, and mixtures thereof.
Also, it has been found particularly to be effective when the alkaline electrolyte comprises a mixture of sodium hydroxide, potassium hydroxide, lithium hydroxide, and an acid chose from the group consisting of: boric acid, phosphoric acid, and mixtures thereof.
When boric acid is present, it has a concentration of 0.6 to 1.3 moles per liter.
The stoichiometric excess of alkali hydroxide is between 2.0 moles and 4.0 moles.
Also, lithium hydroxide is present in the amount of about 0.2 moles.